Gregory S. Hancock

River incision into bedrock: Mechanics and relative efficacy of plucking, abrasion, and cavitation

Improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed. We present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion (e.g., plucking, abrasion, cavitation, solution) is a strong function of substrate lithology, and that joint spacing, fractures, and bedding planes exert the most direct control. The relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks, and their spatial distributions. We find that plucking dominates wherever rocks are well jointed on a submeter scale. Hydraulic wedging of small clasts into cracks, bashing and abrasion by bedload, and chemical and physical weathering all contribute to the loosening and removal of joint blocks. In more massive rocks, abrasion by suspended sand appears to be rate limiting in the systems studied here. Concentration of erosion on downstream sides of obstacles and tight coupling between fluid-flow patterns and fine-scale morphology of erosion forms testify to the importance of abrasion by suspended-load, rather than bedload, particles. Mechanical analyses indicate that erosion by suspended-load abrasion is considerably more nonlinear in shear stress than erosion by plucking. In addition, a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued. Cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive, unjointed rocks that is otherwise attributed to suspended-load abrasion. Direct field evidence of cavitation erosion is, however, lacking. In terms of the well-known shear-stress (or stream-power) erosion law, erosion by plucking is consistent with a slope exponent ( n ) of ∼2/3 to 1, whereas erosion by suspended-load abrasion is more consistent with a slope exponent of ∼5/3. Given that substrate lithology appears to dictate the dominant erosion process, this finding has important implications for long-term landscape evolution and the models used to study it.

Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau

2007 American Geophysical Union. Densmore, A. L., M. A. Ellis, Y. Li, R. Zhou, G. S. Hancock, and N.Richardson, (2007), Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the TibetanPlateau, Tectonics, 26, TC4005, 10.1029/2006TC001987. To view the published open abstract, go tohttp://dx.doi.org and enter the DOIUse policy

Numerical modeling of fluvial strath-terrace formation in response to oscillating climate

Many river systems in western North America retain a fluvial strath-terrace rec ord of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marine δ 18 O rec ord. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation. Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.

Estimates of the rate of regolith production using 10Be and 26Al from an alpine hillslope

Abstract The production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material. We use concentrations of the cosmogenic radionuclides (CRNs) 10 Be and 26 Al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the Wind River Range, WY. These calculations are based on a theoretical model which we develop here. This model shows that it is important to consider dissolution of regolith in regolith production and in basin-averaged erosion rate studies. Rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 Be and 26 Al are 14.3±4.0 and 13.0±4.0 m Ma−1, respectively. Rates of production of regolith deduced from 10 Be concentrations in regolith-mantled bedrock support the rates deduced from regolith concentrations. In the alpine environment examined here, the rate of production of regolith beneath ∼90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces, which Small et al. [Small, E.E., Anderson, R.S., Repka, J.L., Finkel, R., 1997. Erosion rates of alpine bedrock summit surfaces deduced from in situ 10 Be and 26 Al . Earth and Planetary Science Letters 150, 413–425] previously documented. Rock-mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces. Because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope, the regolith volume flux must be proportional to the local slope of the hillside. Therefore, our results are consistent with Gilberts [Gilbert, G.K., 1909. The convexity of hilltops. Journal of Geology 17, 344–350] steady state hillslope hypothesis. If tor height and the difference between rates of weathering on bare and regolith-mantled rock provide a fair estimate of the age of summit flats, steady-state hillslope conditions have been attained in less than several million years.

Dating fluvial terraces with 10Be and 26Al profiles: application to the Wind River, Wyoming

Abstract Fluvial strath terraces provide a record of river incision and the timing of climatic perturbations to the fluvial system. Dating depositional surfaces like terraces that are older than the range of 14 C , however, is difficult. We employ a cosmogenic radionuclide (CRN) profile technique that addresses a major problem of CRN dating on such surfaces: nuclide inheritance. By measuring 10 Be and 26 Al profiles, we constrain the exposure age and the mean CRN inheritance for the deposit. The CRN profile also yields a self-check on the assumptions underlying the method. We report our attempts to date terraces along the Wind River, WY. Like many sequences of western North American fluvial terraces, these are inferred to reflect oscillation between glacial and interglacial conditions in the headwaters. Previous dating of some of these terraces and the associated terraces and glacial deposits makes this a unique location to compare dating methods. Dates from five sites along the Bull Lake-glacial correlative terrace (WR-3) are ∼118–125 ka, which agrees with dates on Bull Lake-age moraines and independent age estimates on the terrace, and is consistent with the model of terrace–glacial relationship. CRN inheritance is significant and highly variable, requiring it be considered despite the additional sampling complexity. Assuming all inheritance in WR-3 deposits arises during exhumation in the headwaters, we obtain minimum mean rates of exhumation of ∼13–130 m/My for the source rocks. Alternatively, assuming the CRNs are inherited during clast transport, the time of fluvial transport from source to terrace is >∼10 ka; it increases downstream and is lower for sand than cobbles. The CRN ages for older terraces (WR-7=∼300 ka and WR-15=∼510 ka) are lower by ∼50% than previous estimates based on tephrochronology; the most plausible explanation is eolian deflation of a once thicker loess cover on the terrace surfaces. Mean thicknesses of loess of ∼0.5–1.5 m are required to reconcile these concentrations of CRN with the previous estimates of age. Difficulty in dating the older terraces emphasizes that geologic caution, independent estimates of age, and multiple sample sites should still be part of dating depositional surfaces with CRNs, even when employing the inheritance-correction technique.

Summit erosion rates deduced from 10Be: Implications for relief production in the central Appalachians

We have measured erosion rates using 10 Be from bare-bedrock surfaces exposed at high elevations at Dolly Sods, West Virginia, a classic Appalachian paleoperiglacial plateau. The mean erosion rate from nine samples is 5.7 m/m.y., signifi cantly lower than previously estimated periglacial erosion rates in this region. Measured bare-bedrock erosion rates likely represent the rate at which the highest portions of this broad upland are being lowered. Fluvial incision rates measured in the region over similar time scales are ≥2 times faster, suggesting relief is increasing in this portion of the Appalachians. This observation of increasing relief is inconsistent with prior work suggesting that the central Appalachian landscape is in dynamic equilibrium or currently decreasing in relief. We hypothesize that late Cenozoic climate change has accelerated fl uvial incision rates, creating a disequilibrium landscape with growing relief with hillslopes undergoing adjustment to increased fl uvial incision rates.

Modeling the effects of weathering on bedrock‐floored channel geometry

[1] Field and modeling studies suggest that bedrock channels equilibrate to base‐level change through geometry and slope adjustment to imposed discharge, sediment supply, and substrate erodibility conditions. In this study we model the influence of bedrock weathering on channel geometry and slope as mean peak discharge (Qm) and uplift rate (U) vary. We find that channels in which weathering is allowed to increase erodibility are wider, deeper, and less steep than nonweathering channels with the same initial conditions. While fixed erodibility channels maintain similar width/depth ratios regardless of Qm or U, the width/depth ratio of weathering channels is sensitive to uplift rate. At low uplift rates, weathering outpaces erosion, and channels obtain similar width/depth ratios but are wider and less steep than fixed erodibility channels with equal initial conditions. At high uplift rates, erosion outpaces weathering and erodibility remains near the unweathered value, with channel shape and slope nearly identical to a fixed erodibility channel with equal initial conditions. Weathering channels differ most from fixed erodibility channels at intermediate uplift rates, with greater width/depth ratios and lower slopes than fixed erodibility channels with the same initial conditions. Our results support the hypothesis that cross‐channel variations in erodibility created by weathering may be an important control on channel geometry and provide guidance for further testing of this hypothesis in natural systems.

Variability of rock erodibility in bedrock‐floored stream channels based on abrasion mill experiments

We quantify variations in rock erodibility, Kr, within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in Utah and basaltic bedrock in the Hawaiian Islands. Samples were taken from the thalweg and channel margins, the latter at a height that is inundated annually. For each sample, a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth. Erosion rate data from these experiments shows two things. First, the erosion rate from channel margin samples is greater than for thalweg samples, with the greatest difference observed for the rock surface that was exposed in the stream channel. Second, erosion rate decreases with depth beneath the original rock surface, by an order of magnitude in most cases. The erosion rate becomes steady at depths of 1–3 mm for channel margin samples and 0.1–0.4 mm for thalweg samples. Because only rock properties and microtopography vary throughout the sequence of mill experiments, these results suggest that Kr of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that Kr decreases over depths of ~1 mm. The simplest explanation for these patterns is that Kr is enhanced, at the bedrock surface and along the channel margins, due to the effects of weathering on rock strength and surface roughness. We hypothesize that a balance exists between weathering-enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg.

Developing quantitative skills activities for geoscience students

Teaching quantitative skills is one of the most challenging and important aspects of teaching geoscience. Quantitative skills are essential for Earth science students and citizens alike, and these skills have been deemed a critical goal for U.S. undergraduate education [National Science Foundation, 1996]. Public policy decisions increasingly are made and explained to the public based on data presentation, numerical models, statistics, and numbers. Introductory Earth sciences are among the primary courses chosen by nonscience students to fulfill science requirements and thus provide an important vehicle for teaching these essential skills. Courses in the geoscience major are equally important in introducing students to the quantitative analysis that is increasingly central to the discipline. It is therefore essential that adequate training be provided to Earth science majors and nonmajors in quantitative techniques ranging from simple graph and data interpretation to more sophisticated techniques such as numerical modeling.

Weathering and abrasion of bedrock streambed topography

Our framework for understanding morphodynamic feedbacks in bedrock rivers is built upon the assumption that rock erodibility is reasonably uniform at the sub-reach scale. Here, we demonstrate that climate-controlled rock weathering combined with bedload abrasion can produce systematic spatial variations in erodibility across bedrock streambed topography. Rock strength data from five channel reaches across the Big Island of Hawaiʻi show that upstream-oriented rock surfaces are stronger than downstream-oriented surfaces on the same bedrock protrusion. Moreover, the overall strength of these protrusions correlates with local mean annual precipitation rate, demonstrating climatic control of streambed erodibility. Comparing inferred field abrasion rates with experimental flume measurements, we demonstrate that abrasion rates scale exponentially with the orientation of local bed topography relative to streamflow, independent of weathering. However, the spatial variability in abrasion rate across bedrock protrusions is significantly reduced in the field, where large spatial variations in erodibility occur due to weathering. The methods presented here provide a straightforward field-based approach for evaluating the potential influence of weathering on abrasion in bedrock rivers.